(1) Field of the Invention
The present invention generally relates to a method and an apparatus for processing an image corresponding to a radiographic pattern, and more particularly to a method and an apparatus for processing an image corresponding to a radiographic pattern, which can be applied to an image processing on an X-ray photograph obtained by a digital X-ray medical equipment.
(2) Description of Related Art
Medical photographs such as X-ray photographs are used for diagnosing diseases. The X-ray photographs are obtained in accordance with the following procedure. The X-ray applied to a subject is projected onto a fluorescent material (a fluorescent screen) after passing through the a subject, so that the fluorescent material radiates a visible light. The visible light from the fluorescent material is projected onto a silver film. Then the silver film is developed, and the X-ray photograph is obtained. The above procedure is often referred to as a screen/film method (herein after referred to as S/F method).
When the number of the X-ray photographs increases, large space is needed to keep X-ray photographs. In addition, it is difficult to find an X-ray photograph from among a large number of X-ray photographs. Thus, the following method has been proposed: images on X-ray photographs are converted to digital image information by a film reader, and the digital image information is recorded in an optical disk. When digital image information is stored in an optical disk, even if an X-ray photograph is obtained under unsuitable conditions, a suitable image can be obtained by carrying out an image processing with respect to the digital image information.
Conventionally, in an X-ray imaging system having high sensitivity and a high resolution, a photostimulable phosphor (an acceleratable phosphor) has been used. A detailed discussion of a radiation imaging system using the photostimulable phosphor is given in U.S. Pat. No. 3,859,527. The photostimulable phosphor used in the system disclosed in this patent has a characteristic in which part of the energy supplied thereto through an irradiation, such as that of X-rays, is stored in the photostimulable phosphor. This state, where energy is stored in the photostimulable phosphor, is stable, so that the state is maintained for some time. When a first light, such as an acceleration light, is projected onto the photostimulable phosphor having the above state, a second light is radiated thereby, corresponding to the energy stored in the photostimulable phosphor. The second light is often referred to as an accelerated phosphorescence. The first light is not limited to a visible light, and can have a frequency falling within a wide range between an infra-red and an ultra-violet. The second light also can have a frequency falling within a wide range between infra-red and ultra-violet. The frequency of the second light depends on a material of the photostimulable phosphor. The second light (an electromagnetic wave) is converted into an electrical signal by a photoelectric conversion device. Then, digital image information is obtained based on the converted electrical signal.
FIGS. 1 and 2 respectively show a flow chart and a film reader for obtaining a digital image from an X-ray film obtained in accordance with the S/F method described above.
Referring to FIG. 1, an X-ray film (an X-ray photograph) is obtained in accordance with the S/F method, in step 100. The X-ray film obtained in step 100 is supplied to a film reader, so that a digital image is generated thereby, in step 110. In the film reader shown in FIG. 2, the X-ray film supplied thereto is fed, by feed rollers 2, through a feed path 1 to a reading area A.sub.r. While the X-ray film passes through the reading area A.sub.r, a laser beam emitted from a laser scanning system 3 scans the X-ray film in accordance with a raster scanning method. The laser beam transmitted through the X-ray film is projected onto a photo detecting element array 5. The photo detecting element array 5 carries out photoelectric conversion so that digital image information corresponding to the converted electrical signal is obtained.
The digital image information is stored in a memory, in step 120. Then an image corresponding to the digital image information stored in the memory is displayed on a display unit, in step 130.
In a case where an image on the X-ray film is printed on a photographic paper, the photographic paper is supplied to the film reader 6. In this case, the laser beam reflected by the photographic paper is projected onto a photo detecting element array 4. Digital image information is obtained via the photo detection element array 4.
Examples of a system in which digital image information is obtained through use of the photostimulable phosphor are shown in FIGS. 3 and 4. A system shown in FIG. 3 has separate radiography and reading part parts; a system shown in FIG. 4 has them integrated.
Referring to FIG. 3, an photostimulable phosphor sheet (or plate) 15 is detachably mounted in a radiography stand 14. In a state where a subject 12 stands so as to face the photostimulable phosphor sheet 15, an X-ray 13 radiated by an X-ray generator 11 is supplied to the subject 12. Then the X-ray transmitted through the subject 12 is projected onto the acceleratable sheet 15, so that a latent image corresponding to a energy pattern stored in the photostimulable phosphor sheet 15 is formed on the photostimulable phosphor sheet 15. After the radiography is completed, the photostimulable phosphor sheet 15 is detached from the radiography stand 14 and supplied to a receiving part 17 of a reading unit 16. The photostimulable phosphor sheet 15 is fed from the receiving part 17 to a reading process block 19 via a feed path 18. In the reading process block 19, the first light scans the photostimulable phosphor sheet 15, and then image information is obtained based on accelerated phosphorescence (a second light) radiated, due to the scanning of the first light, from the photostimulable phosphor sheet 15. The image information obtained by the reading process block 19 is supplied to an image processing block 23 via a image transference path 22. The image processing block 23 carries out a predetermined image processing with respect to the image data supplied thereto. Then the image information output from the image processing block 23 is supplied to a display block 25, so that the image information is displayed on a CRT display unit or a printer outputs a hard copy of the image information, in the display block 25. After the process in the reading process block 19 is finished, the photostimulable phosphor sheet 15 is fed to an erasure block 20 via the feed path 18. In the erasure block 20, the first light is projected onto the photostimulable phosphor sheet 15 so that the energy stored therein is completely radiated. That is, the residual latent image is erased from the photostimulable phosphor sheet 15. After that, the photostimulable phosphor sheet 15 is fed to an ejection part 21. The photostimulable phosphor sheet 15 is then returned from the ejection part 21 of the reading unit 16 to the radiography stand 14.
The photostimulable phosphor sheet 15 may be provided in a magazine or a cassette.
In a system shown in FIG. 4, the photostimulable phosphor sheet 15 is set in a standing radiography unit 26. In a state where the subject 12 stands so as to face the photostimulable phosphor sheet 15 in the standing type radiography unit 26, the X-ray radiated by the X-ray generator 11 is supplied to the subject. After the latent image is formed on the photostimulable phosphor sheet 15 by supplying X-ray to the subject, the photostimulable phosphor sheet 15 is processed in the same manner as that in a case shown in FIG. 3.
The digital image information obtained by either the system using the film reader or the system using the photostimulable phosphor sheet (plate) is stored in a digital recording medium, such as an optical disk. Thus, a large space is not needed to store image information of the X-ray photographs. In addition, image information of an X-ray photograph out of a large number of X-ray photographs can be easily retrieved.
Conventionally, large number of image processing methods of digital images have been reported. In addition, an image processing of digital X-ray photograph also has been proposed.
A spatial frequency processing carried out in accordance with a formula (1) is well known, EQU Q=S.sub.ij +K.multidot.(S.sub.ij -S.sub.m) (
where S.sub.ij is image data of a central pixel in an unsharp mask which is an n.times.n matrix as shown in FIG. 5, S.sub.m is an average of pixel data in the unsharp mask, K is an enhancement coefficient, and Q is image data obtained by the spatial frequency processing.
A spatial frequency processing in which the enhancement coefficient K is variable has been proposed. This type of spatial frequency processing is carried out in accordance with, for example, the following formula (2), EQU Q=S+f(S-S.sub.m).multidot.(S-S.sub.m) . (2)
where f(S-S.sub.m) is an enhancement coefficient which is a function of (S-S.sub.m). That is, the enhancement coefficient f(S-S.sub.m) has a value in accordance with image data of each pixel.
A spatial frequency processing referred to as a Wallis filter has been well known. The spatial frequency processing is carried out in accordance with the following formula (3) using a standard deviation value .sigma., EQU Q=S+(A/.sigma.+B) (S-S.sub.m) . (3)
where A and B are constants. In this type of spatial frequency processing, the enhancement coefficient is varied with respect to the standard deviation value .sigma..
In an X-ray photograph of a thorax, which photograph is obtained in accordance with the S/F method, there are shown white vascular tracts in a appropriately black lung region. A centrum, a diaphragm region and a heart region have a low density (close to white). The centrum appears slightly in the X-ray photograph, and information of regions other than the lung region is hardly to be obtained from the X-ray photograph. When the X-ray photograph of the thorax is taken so that contrast in regions other than the lung region is improved, the density of the lung region is increased. In this state, since the dynamic range of a density distinguished by the human eye is narrow, the vascular tracts in the lung region become indistinct.
In the X-ray photograph, regions other than the lung region are indistinct. However, there can be diseases in a thoracic vertebrae, heart, ribs, a lung behind a diaphragm. Thus, it is desired that a diagnosis of all anatomical regions can be performed based on one X-ray photograph.
Image processing can be applied to a digital X-ray photograph. Due to the image processing in which an image is enhanced, the thoracic vertebrae region, the heart region and the lung region can become slightly more distinct. On the other hand, as the vascular tracts in the lung region are enhanced in the same manner as the lung region, it becomes hard to see the lung region due to the enhanced vascular tracts. In this case, it is hard to diagnose diseases, such as incipient cancer, which are distinct in a digital X-ray photograph before applying the image processing.
Thus, a method in which two thorax images are formed on one film has been proposed. The first image corresponds to a normal X-ray photograph to which the image processing is not applied or is weakly applied. The second image is an image to which the image processing is strongly applied so that edges are enhanced. The lung region is diagnosed by using the first image. The centrum, the diaphragm, and the heart region are diagnosed by using the second image. According to this method, as the two images are formed on one film, a size or each image is less than that of the actual thorax. When each image is life size, the film having two images becomes exceedingly large. Thus, a cost of the film is increased.
A method has been proposed for obtaining an X-ray photograph in which a mediastinum region, and a heart-diaphragm region are distinct in the same condition as the lung region. In the S/F method, for example, compensation filters are used. In this case, an X-ray absorption layer having a first thickness is provided on the lung region, an X-ray absorption layer having a second thickness less than the first thickness is provided on the heart-diaphragm region, and an X-ray absorption layer having a third thickness less than the second thickness is provided on the mediastinum. This method is effective, so that not only the mediastinum but also other anatomical regions can be accurately diagnosed. This method is a type of gradation processing.
However, in the method using the compensation filters, the compensation filters, each having a size corresponding to each subject, must be provided to each subject. An operation for providing the compensation filters to each subject is troublesome.
Japanese Patent Laid Open Publication No. 61-35069 discloses the following image processing of a digital X-ray photograph which can obtain the same effects as the above compensation filters used in the S/F method.
A digital X-ray photograph obtained by using the photostimulable phosphor is displayed in a CRT. In a case of, for example, a thorax X-ray photograph, a border between the diaphragm having a low X-ray transmittance and a lung region having a high X-ray transmittance is recognized by visual observation on the CRT. Then, a predetermined value (signal) is added to image data of each pixel in a region having a low X-ray transmittance. In addition, the predetermined value is subtracted from image data of each pixel in a region having a high X-ray transmittance.
It is assumed that the size of the photostimulable phosphor sheet and the size of each subject are approximately constant. Thus, a standard pattern having regions each formed of pixels in which the density (image data) should be corrected has been previously stored in a computer. Then the predetermined value is added to a density of each of the pixels in accordance with the standard pattern.
However, the above image processing has the following disadvantages.
First, a border between adjacent anatomical regions must be recognized by visual observation on the CRT. Thus, it is hard to rapidly carry out the image processing.
Second, the size of the photostimulable phosphor sheet can be constant, but it is unwarranted to assume that the size of each subject is constant. That is, sizes between children and adults, men and women, fat people and slender people respectively differ from each other. In addition, it is difficult to set each subject at a constant position with respect to the photostimulable phosphor sheet. Thus, there is a case where each border between adjacent anatomical regions formed in each radiographic image differs from each border between adjacent anatomical regions in the standard pattern. In this case, a clear digital radiographic image can not be obtained.